1. Field of the Invention
This invention relates to an optical system of a copying machine, and more particularly to an optical system of a copying machine in which a multi-layered interference film for adapting the spectral wavelength characteristic of an original light to the spectral sensitivity characteristic of a photosensitive medium is applied to a surface of a transmitting type lens system.
2. Description of the Prior Art
As described in our prior Application Ser. No. 964,986, now U.S. Pat. No. 4,218,134, which discloses a multi-layered interference film provided on a reflecting type projection lens, if, for example, a halogen lamp as a light source and a CdS photosensitive medium as a photosensitive medium are used in the optical system of a copying machine (in the present specification, the light source and the photosensitive medium with the other optical members are referred to as the optical system for the sake of convenience), red characters or figures depicted on an original are copied only thinly or not at all. Such phenomenon occurs because, at the filament temperature of about 3000.degree. C. usually used when a halogen lamp is used, the light-emission energy thereof has a maximum value in the infrared range of 800-900 m.mu. and is uniformly decreased toward the short wavelength side, and the spectral sensitivity of the CdS photosensitive medium is higher in the red to near-infrared, with a result that the exposure of red portions becomes excessive as compared with the exposure of blue or green characters or figures on the original. When non-conformity so occurs between the wavelength characteristic of the object light and the sensitivity characteristic of the photosensitive medium, it is usual to dispose a filter in or adjacent to an image forming lens and cut the unnecessary wavelength range, but in an apparatus wherein no filter is removably mounted, it is also possible that the thin film provided on the substrate of the filter is directly applied to a surface of the image forming lens. For such thin film, an interference thin film which readily provides the desired characteristic has been widely used. However, if a multi-layered interference thin film having a property similar to that of the color resolving dichroic mirror of a color television camera or the cold mirror used in an illuminating system is applied to a surface of the projection lens so that light of necessary wavelength is transmitted therethrough while light of unnecessary wavelength is reflected and eliminated thereby, the transmission factor and copying performance may be maintained, whereas a preferred result has not been obtained when an experiment has been carried out by using a multi-layered interference thin film in which the center wavelength of the conventional dichroic mirror has been moved. Through the examination of this experiment, problems in the spectral transmission factor characteristic of the multi-layered interference film in a transmitting type lens system have been extracted as follows:
(1) If red light of 550 m.mu. or more, which is an unnecessary wavelength, is sharply cut, the contrast of red is enhanced but the contrast of blue, green or cyanic color is reduced. This is a point asserted in a multi-layered film having thirteen layers as disclosed, for example, in Japanese Laid-open Patent Application No. 60142/1977.
(2) Any multi-layered film which does not have an average transmission factor of at least 70% for blue and green lights which are in the wavelength range of 400 m.mu. to 500 m.mu. is unsuitable.
(3) The transmission factor for red and infrared lights which are in the wavelength range of 600 m.mu. to 800 m.mu. should desirably be 50% or less, but too low a transmission factor for these lights would cause reduced reproducibility of blue and cyanic colors.
Japanese Laid-open Patent Application No. 39146/1978 suggests attachment of a wavelength-selective transmitting film to a surface of a transmitting type projection lens but does not disclose the concrete construction of such film.